I have an onboard Onan 5500 gas and would never have a 5'er without one. Yes, I do run it every month without fail for one hour under load and I use non-ethanol gas and I still put stabilizer in the gas. At a moderate load with two a/c's it will run 20 hours on the 10 gallon built in tank. If it is real hot out the 20 hours is a little bit less or more if cooler. I carry two 5 gallon gasoline cans with me so I can fill it up if necessary when boondocking.
After just over 4 years the generator has ever failed or caused a problem. I change the oil and oil filter once a year as well as check the air filter and replace as needed (probably before really needed, but then that is just me). At this point it has 216 hours on the clock.